To effectively drive a brushless direct current (BLDC) motor, a motor control system requires accurate information on the position of the rotor in relation to the stator. Sensors such as Hall effect sensors may be used to sense rotor position. However, the use of such sensors increases cost and weight, decreases reliability, and subjects the motor to temperature limitations imposed by the operational limitations of the sensors.
A form of sensorless control of (BLDC) motors is known; it typically involves estimation of the rotor speed and/or position based on induced electromotive force (EMF) or back-EMF occurring in an non-energized stator winding. One known technique involves monitoring zero voltage crossings in the EMF generated in the non-energized (non-driven) motor winding in order to determine the position of the rotor. The position of the rotor is then fed back to a commutating circuit to provide a proper commutation sequence to stator windings. Examples of such motors are disclosed in U.S. Pat. No. 5,057,753 to Leuthold et al. and U.S. Pat. No. 5,231,338 to Bulgarelli et al. Difficulties are however encountered at high speed applications with known techniques of monitoring zero voltage crossings in the EMF generated in the non-energized motor winding. Improvement in sensorless control is therefore desirable.